42 



ORGANOGRAPHY. 



cle, becomes of considerable thickness, as in the upper surface of 



Fig. 87. 



Fig. 87. Section of a leaf of Cycas 

 revoJuta, showing the epidermis 6, 

 covered bv a thickened cuticle a. 

 After Schleiden. 



the leaves of Cycas, &c. (Jig. 87). 

 The nature of cuticle has given 

 rise to much discussion, and cannot 

 be said to be as yet accurately 

 determined. By most observers 

 it is regarded as an excretion from 

 the epidermal cells, which has be- 

 come hardened on their surface; 

 others again, regard the cuticle as 

 the permanent original outer wall 

 of the parent cells of the epider- 

 mis, which has become chemically altered by the action of the 

 air. Mitsclierlich, again, considers it as a substance analogous 

 to cork, and thus adapted to prevent moisture from penetrating 

 the tissues beneath. "We shall refer again to its nature, when 

 speaking of the Intercellular substance with which it is supposed 

 to be analogous. A homogeneous membranous layer resem- 

 bling, if not actually identical with cuticle, is found upon the 

 surface of plants living under Avater; and upon that of the Algce, 

 Lichens, and Fungi, which have no true epidermis. The outer 

 membrane of most spores and of pollen grains is of a similar 

 nature to the cuticle, which fact, as argued by Schacht, is a strong 

 evidence of the latter being an excretion from the epidermal cells, 

 c. Stomata or Stomates. — These are orifices situated between 

 some of the epidermal cells, opening into the intercellular cavities 

 beneath, so as to allow a free communication between the in- 

 ternal tissues and the external air {figs. 90 and 91, s)\ hence 

 they are also commonly called breathing pores. These orifices 

 are surrounded by cells of a different form from those of the 

 epidermis, and which usually contain some chlorophyll granules. 

 There are generally but two cells surrounding the orifice, 

 Fig. 88. Fig. 89. 



.i 



1 



Fiu- 88. Epidermis of the Lily, with etomata Fifj. 89. Stoma of J/or- 



cltantia poli/morplia (IlepaticacesB). After Carpenter. 



which are commonly of a more or less semilunar form (Jigs. 76 

 and 88), so that the whole has some faint resemblance to the 



